Two-component type developer comprising a toner and a carrier has the advantages that a polarity and a charge amount of the toner can be controlled to some extent and that the selection of color for the toner can be widened.
In this kind of a developer, a carrier is composed of a core material covered with a resin in order to control frictional electrification, prevent deterioration of the carrier and damage to the surface of a photoreceptor, lengthen a shell-life of the developer and maintain quality of a copied image.
In a high speed copier developed recently for repeated and frequent use, an overcoat layer for covering the carrier is liable to be easily peeled off and enable no prescribed effects to be achieved. Therefore, in order to improve an abrasive resistance of a resin layer, there are proposed the methods in which a thicker layer is provided and in which an overcoat layer is strengthened by mixing therein grains less liable to be abraded (a filler), disclosed in Japanese Patent Publication Open to Public Inspection No.73631/1985.
However, a thicker layer is liable to increase a production time in a production process and decrese a yield in a grain-forming step.
A spray coating method and a dipping method are used in order to incorporate a filler into an overcoat layer. It is difficult, however, to disperse the filler stably in a resin solution, and an abrasive property and a friction electrification are varied to a large extent by lot. Besides, the isolated fillers stick to a photoreceptor and damage it, which in turn results in causing a deteriorated image, fogging and inferior cleaning. Further, the filler itself is liable to generate a spent.
In the method where an impact force is repeatedly applied to a mixture of a core material and a resin particle to thereby cover the core material with the resin particle, it is possible to increase a layer thickness of a carrier by increasing the size of the resin particle and the impact force to thereby increase an amount of the resin coated on the carrier by one dry coating. However, it is difficult to make a uniform layer by this method. Further, where ferrite is used for the core material, the increased impact force causes abrasion and crush, so that the carriers having different particles sizes are liable to be formed and a sieving process is necessary for removing generated fine particles.